Field of the Invention
The invention relates to a device for switching a direct current in a branch of a DC voltage network node, having a permanent current path, in which a mechanical switch is arranged, a switched current path which bridges the permanent current path and in which a power switching unit is arranged, which power switching unit has power semiconductor switches which can be switched on and off and is set up to interrupt a short-circuit current in the event of a fault, and a longitudinal voltage source for generating a back-emf in the mesh formed by permanent current path and switched current path.
A device of this type is already known from WO 2011/057675 A1, for example. That document describes a DC voltage switch which has a permanent current path with a mechanical switch and a disconnection branch, which is connected in parallel with the permanent current path. A power switching unit in the form of a series circuit composed of power semiconductor switches is arranged in the disconnection branch, a freewheeling diode being connected in each case so as to be reverse-biased in parallel with said power semiconductor switches. The power semiconductor switches and freewheeling diodes of the power switching units are arranged back-to-back, that is to say the disconnectable power semiconductor switches are arranged in series, wherein, for each power semiconductor switch, a corresponding power semiconductor switch with opposite forward direction is provided. In this way, the current can be interrupted in both directions in the disconnection branch by the power switching unit. In the operating or permanent current path, in addition to the mechanical switch, an electronic auxiliary switch is also arranged in series with the mechanical switch. During normal operation, the current flows via the operating current path and thus via the electronic auxiliary switch and via the closed mechanical switch since the many power semiconductor switches of the disconnection branch represent an increased resistance to the direct current. To interrupt, for example, a short-circuit current, the electronic auxiliary switch, which can also be referred to as longitudinal voltage source, is transferred into its blocking position. As a result of this, the resistance in the operating current path increases, with the result that the direct current in the disconnection branch is commutated. The mechanical disconnection switch can therefore be opened with no current flowing. The short-circuit current conducted via the disconnection branch can be interrupted by the power switching unit. To accommodate the energy which is stored in the DC voltage network and is to be built up during switching, arresters are provided which are in each case connected in parallel with the power semiconductor switches of the disconnection branch.
Further DC voltage switches for switching short-circuit currents at high voltages are disclosed in DE 694 08 811 T2 and in U.S. Pat. No. 5,999,388.
The device mentioned at the outset has the drawback that said device cannot be used for load flow control since the losses occurring at the longitudinal voltage source then occur over long periods of time or even constantly. This is not sustainable from an efficiency or ecological point of view.